(1) Field of the Invention
The invention relates to a delta-modulation encoder for information signals, comprising a first input circuit provided with a cascade arrangement of a decoder and a comparison circuit, said information signals being applied to said first input circuit, which in response thereto produces an output signal which is applied to a quantizing device which is controlled by a pulse generator and which comprises an output circuit at which the delta modulation output pulses are produced; said output pulses being further applied through a first feedback circuit to the first input circuit; compensation means comprising an integrating network, a second input circuit which is coupled to the output circuit of said quantizing device and an output circuit coupled to said comparison circuit.
Here the term delta-modulation encoder must also be understood to mean a delta-sigma-modulation encoder which differs from the delta-modulation encoding arrangement in the place of the decoder in the input circuit with respect to the comparison circuit.
Encoding arrangements of the type described and which are in use nowadays, are all arranged for encoding information signals varying over a large dynamic range, such as speech signals. For high accuracy encoding of such signals with a comparatively low sampling frequency these encoding arrangements comprise dynamic control means which usually consist of a step-size control circuit included in the first feedback circuit and through which the output pulses of the quantization device are applied to the input circuit. This step-size control circuit varies the energy contents of the pulses fed to it between a predetermined minimum and a predetermined maximum value. The energy contents of these pulses is, for example, proportional to the level of the information signals and is usually derived by the step-size control circuit from the output pulses of the quantizing device.
(2) Description of the Prior Art
A coding arrangement of the type described and which comprises the above-mentioned dynamic control means has already been described in the prior U.S. Pat. No. 3,831,092. In this encoding arrangement the compensation means are constituted by a second feedback circuit which includes said integrating network and to which the output pulses of the quantizing device are applied. The output circuit of the second feedback circuit is coupled with said comparison circuit. In this prior art arrangement the decoding arrangement in the input circuit is constituted by the integrating network and the cut-off frequency of the integrating network in the second feedback circuit is considerably lower than that of the integrating network in the input circuit. The output pulses of the quantizing device are applied to the integrating network in the second feedback circuit with a predetermined energy content. As explained in said U.S. Pat. No. 3,831,092 this second feedback circuit compensates for drift voltages which are produced in the various elements of the encoding arrangement. On the one hand the use of this compensation circuit increases the dynamic range by approximately 10-15 dB and on the other hand an improvement of the reproduction quality is obtained for small values of the information signal, that is to say of information signals which are of the order of magnitude of said drift voltages.
Although this prior art encoding arrangements considerably improves the encoding accuracy of small values of the information signals, no improvement, even a deterioration, is found in the encoding accuracy of information signals with small amplitudes when they occur after the information signal has rapidly decreased from a high value to a low value, even when this low value is still considerably larger than said drift voltages.